Sound recording



July 19, 1932- H. c. HARRISON 1,868,321

SOUND RECORDING Filed Nov. 4. 1929 //vv/v 70/? H C HARRISON Pmmd M 19,1932 UNITED s'rAras PATENT OFFICE.

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eretofore various attempts have been made to eliminate scratch, some ofwhich involved the use of filters which were designed to discriminateagainst that part of the frequency range in which the major portion ofthe scratch wasvbelieved to occur. I

It has also been proposed to shift the frequency band to be recorded'outof the range wherein the scratch is most objectionable and then, bymeans of filters and demodulators in the reproducing system, toeliminate the scratch noise in reproduction. A system of this kind'isdescribed in Patent 1,707,260, granted to C. H. Fetter, April 2, 1929.Obviously both of these schemes involve the use of extra apparatus inthe recording system.

In the present invention, however, it is proposed to eliminate theseundesired sounds by removing the condition which causes it, that is tosay, by burnishing or smoothing out the extraneous irregularitiesv inthe grooves after the original record is made.

It is well known in the art of burnishing that for a satisfactoryfinish, the work should be done at fairly high velocity and with aenough the permissible initial steady pressure on the wax will be lar cwith respect to the alternatin .reactions (16 to the groove variationsso t at the stylus point maintains reasonablg constant pressure againstall portions of t e groove. It has been found that after one or moreplayings of this kind that, contrary to previous experience with thebest known reproducers, the scratch noise heard upon reproducing therecord in the normal manner is very appreciably reduced due, it isbelieved, to the burnishing or polishing effect of the stylus upon thegroove surface. Since as already explained this treatment affects onlythe surface of the groove, the original amplitude response of therecorded sounds is not impaired. In some cases, depending upon suchconsiderations as the impendance of the particular vibrating systemused, the range of frequencies recorded and the physical characteristicsof the material, it may be desirable to burnish at higher or lowerspeeds than used in recording. An increase in the effective burnishingspeed may be obtained when desired by having a system of the properimpedance in which the stylus is rotated at a suitable speed while incontact with the groove. Although this treatment is very useful in caseswhere the original recordsare to be used for playback purposes, it iseven more valuable from a commercial standpoint because of the fact thatthe improvement made in the original is also incorporated in anypressing made from it. Y

A clearer understanding of the invention may be had from a considerationof the following detailed descriptionand the accompanying drawing inwhich Fig. 1 shows a stylus and resilient mounting in the simplest formsuitable for the purposes of the invention and Fig. 2 shows amodification in which the stylus is rotated while in contact with thegroove to be burnished.

While the invention is described with reference to its application torecords of the hill and dale ty e it is of course equally applicable tolateral cut records, the only requirement being that the impedance atthe stylus point and the bearing pressure shall be of the proper valuesand correctly proportioned. For hill and dale recording it has beenshown in a coending application of A. C. Keller, Serial point impedanceover the frequency range of the record. Expressed analytically, thisbecomes where F is the total force, S is the stiffness in dynes percentimeter of the stylus mounting, Q, is the initial deflection incentimeters of the stylus mounting when the stylus is in contact with ablank groove of the record, V is the maximum velocity of the styluspoint in centimeters per second and Z is the stylus point impedance inmechanical ohms, all of which are considered with reference to thestylus point.

The above equation may be written and since for any given record V isdetermined it will be possible to reduce the impedance Z to any desiredvalue within the limits of the equation. These design equations and thereference to the Keller application are given only by way ofillustration as to how suitable vibratory structures may be obtained butthis invention is, of course, primarily concerned with obtaining aburnishing effect and hence is not limited to th use of any particularstructure.

Referring now to Fig. 1, 1 is a sectional view of a hill and dale groovemoving under the stylus 2 in the direction indicated. The stylus'shouldhave almost negligible mass and in the structure shown is thereforecomposed of fibre or other lightweight material and made as short and asslender as practicable. Spring steel wires 3, 3 are looped around andsecured to the stylus and clamped to the structure of the reproducer arm4 to form the necessary elastic support. If this structure is properlydesigned and the record is rotated at the speed most suitable for theparticular record it will be found that the minor surface variations 5in the groove surface willstylus may be provided. In the structure shownfor purposes of illustration in Fig. 2

this takes the form of a very small air motor 6 which is driven by meansof a connection with a compressed air hose 7. The motor is carried uponthe balanced reproducer arm 8 and its s aft 9 extends downwardly throughthe arm and carries a grooved pulley 10 which is connected to a similarpulley 11 on the stylusby a thread-like belt 12. The stylus is held incontact with the groove by means of its own mass and the restoring forceof the reed spring 13 which is clamped in the arm 8 and connected to thecenter of the stylus as shown. Supports 14, 14 drilled to form stylusbearings at one end and secured to the arm 8 at the other hold thestylus in the proper position for the burnishing operation. In otherrespects this device operates in the same manner as the one alreadydescribed.

It should be understood, of course, that the burnishing apparatus shownis drawn to a very large scale for the sake of clarity in illustration.In actual practice the physical dimensions of the stylus and itsmounting would of necessity be very small. In the Keller applicationpreviously referred to, it was shown thata reproducer having aneffective mass of .1 gram was capable of following the groove faithfullyover a range extending far above the upper limit of audibility, but itis found that even this low value of mass is much too great in aburnishing device. Accordingly, the moving system of this invention mustbe proportioned to give an entirely new order of impedance and at thesame time to fulfill the fundamental requirement for remaining incontact with the V groove.

It has been found, for instance, that for soft wax records involving arange of frequencies of the order of 50 to 10,000 cycles, it isnecessary to use a vibratory system having an efiective needle pointmass of .006 gram or less and to operate with a steady force of about 10grams or less, if noticeable improvement with respect to scratch noiseis to be obtained. The wax ordinarily used for an original recording hasa stiffness of about .06 x 10 dynes per centimeteras compared with about.84 X 10 for commercial pressing and it is found that if the steadyforce is much greater than 10 grams, the stylus has a tendency to deformthe groove, particularly for the higher frequencies recorded. It wouldbe desirable to keep the steady force constant in value for all parts ofthe groove in order to obtain uniform burnishing action, but this ofcourse is not possible since the total force on the record is, at anyinstant, the algebraic sum of the steady force due to the deflection ofthe clastic mounting and the alternating forces set up by theundulations in the record groove. However, as previously pointed out, ifthe impedance is made sut'iciently low, as for 'forces will be less thanone-quarter of the steadv force so that reasonably constant pressure isexerted on all parts ofthe groove.

An impedance of this order is obtained when a mounting stiffness of .6 xdynes per centimeter is used with the above massof .006

gram, but it will be understood that even better results may be obtainedbystill further reducing these values.

Various other modifications with respect to exact values of theconstants or the arrangement of the structure will occur to thoseskilled in the art, but the invention is intended to be limited only bythe following claims.

i What is claimed is:

1. The step in the method of producing a grooved phonograph record whichconsists in progressively subjecting all parts of the grooves of theoriginal wax record to a reasonably constant pressure.

2. The step in the method of producing a grooved phonograph record whichconsists in rotating the record ata suitable speed and progressivelysubjecting all parts of the grooves of the original wax sonably constantpressure.

3. Apparatus for burnishing the grooves of high quality phonographrecords in original recording material including a vibration record to arearesponsive system having a stylus and an elastic mounting therefor,the impedance of the system being less than 1,000 mechanical ohms.

4. Apparatus for burnishing the grooves of high quality phonographrecords in original recording material including a vibration responsivesystem having an impedance less than 1,000 mechanical ohms over thefrequeneg range of to 10,000 cycles.

5. pparatus' for burnishing the grooves of a phonograph record inoriginal recording material comprising means for rotating the record, astylus adapted to contact the grooves with a reasonably constantpressure and ing means for rotating the stylus to increase the relativevelocity of the stylus and the groove.

6. Apparatus for burnishing the grooves of a phonograph record inoriginal recordthe record at a speed in excess of the recording speed, astylus ada ted to contact the grooves with a reasonab y constantpressure and means for rotating the stylus to mcrease the relativevelocity of the stylus and .the groove.

7. Apparatus for burnishing the grooves of high quality wax recordsincluding 8." vibration responsive stem of such low mechanical impedancet at the maximum value of the alternating reactions when the system iscontactin a record groove is less than one fourth of e steady forcewhereby the material comprising means for rotating initial pressure ismaintained reasonably constant on all parts of the record grooves.

8. Apparatus for burnishingi the grooves of high q u'alitg wax recordsinc uding means for rotatmg t e records at a speed in excess of therecording speed and a vibration responsive system having an impedanceless than 1000 mechanical ohms over a frequency range of 50 to 10,000cycles.

9. Apparatus for burnishing the grooves of high quality wax recordsincluding a vibration responsive system having an efiective mass lessthan .006 gram cooperating with the record with a steady force less than10 rams.

witness whereof I hereunto subscribe my name this 1st day of November,1929.

HENRY O. HARRISON.

